1. Field of the Invention
This invention relates to microwaveable thermodynamic receptacles, containers, and warming modules. More specifically, this invention relates to containers adapted to maintain their contents at elevated temperatures for extended periods of time. Thermodynamic containers of this invention include a microwave-absorbing composition in heat exchange relationship with the container contents and preferably in a space defined by inner and outer walls of the container. Upon exposure to microwave energy, the composition absorbs energy in the form of microwaves, which is retained in the composition in the form of thermal energy. The thermal energy can then be transferred into the contents of the container, such as a food or beverage, thereby maintaining the contents at an elevated temperature for an extended period of time.
2. Background of the Invention and Related Art
Maintaining the temperature of a food or beverage after cooking or preheating has long been of interest in food and beverage service. In domestic, recreational, field, and commercial environments (among others), maintenance of temperature is desirable when the food or beverage is served some time after cooking or preheating. For example, in commercial environments, food service pans are often placed over a tray of boiling water or over some other heating means to maintain the temperature of the food after preparation. In the home, it is common for a casserole dish to be placed into a wicker basket or wrapped in a cloth towel to insulate the container and maintain the desired temperature of the contents. For the same purpose, electrically heated trivets, or preheated hot-pads, are sometimes used.
Additionally, it is often desirable to be able to consume a food or beverage, prepared earlier, at some location removed from the home, such as at the workplace. In these circumstances, it is often desirable to place food into a portable container which can be easily carried to a separate location where the food can later be consumed. In some environments, such as in remote wilderness locations, or at some sporting events, it can be impossible or impractical to reheat the food or beverage prior to consumption. In these instances, it is both pleasing and convenient to open the container at some later period and find that the food or beverage has been maintained at a desirable temperature.
Portable containers which serve this purpose have been known for years. Early containers designed for this purpose include bottles or other containers, insulated with a foam insulation such as foamable foam polystyrene, e.g., STYROFOAM, or foam polyurethane. Although these containers extend the time a food or beverage may be maintained at an elevated temperature, they do not provide the level of temperature maintenance desired in many instances.
Foam insulated beverage containers have been known for years. A recent advance in double walled foam insulated beverage containers is disclosed in U.S. Pat. No. 5,515,995, to ALLEN et al. This patent discloses a double walled, foam insulated beverage container having a wide base. This patent, and U.S. Pat. No. 3,684,123, to BRIDGES, cited therein, are hereby incorporated by reference as though set forth in full herein.
In general, vacuum insulated bottles are far superior to those insulated with foam. U.S. Pat. No. 3,331,522, to BRIDGES, which is hereby incorporated by reference as though set forth in full herein, describes a vacuum insulated bottle comprising a metal vacuum bottle enclosed in a plastic jacket.
To enhance the ability of the vacuum bottle itself to insulate, attempts have been made to utilize different materials for the vacuum bottle. Although fragile, glass is superior to metal in its lack of thermal conductivity, and thus glass vacuum containers became popular for use in thermally insulated containers.
To further enhance the insulating properties of the glass vacuum container, attempts have been made to line glass vacuum containers with reflective coatings to minimize radiant heat loss. U.S. Pat. No. 3,910,441, to BRAMMING, is illustrative, and discloses a glass vacuum bottle construction in which the interior walls are silver-coated to reduce heat loss.
With the advent of microwave cooking technology and its rapid acceptance and popularity, the need arose for a thermally insulated bottle which could also be used in a microwave oven. However, because metals absorb significant microwave energy, they can become dangerously hot in a microwave oven. Additionally, reflective metals and coatings containing such metals, e.g., silver, can damage the microwave oven magnatron tube by reflecting microwave energy back into the wave guide, and by "arcing" and/or sparking.
A number of attempts have been made at solving the aforementioned problem. For example, U.S. Pat. No. 4,184,601, to STEWART et al., which is hereby incorporated by reference as though set forth in full herein, relates to a microwave-safe vacuum insulated glass container. Instead of a silver lining to reduce radiant heat loss, the annular space of the glass container is substantially filled with finely divided materials which are neither electrically conductive nor absorbent of electromagnetic energy at microwave frequencies. Examples of such materials include finely divided silica and calcium carbonate.
While the vacuum containers which are known in the art are certainly able to conserve the heat of their contents, a continuing need for improvement remains. Most commercially-available vacuum containers known in the art allow a significant loss of thermal energy at a measurable rate (heat loss/unit of time, i.e., thermal efficiency). A need therefore exists for improved thermal efficiency in a microwaveable container.
The present invention enhances the ability of the thermally insulated container to maintain contents at elevated temperatures. Conventional vacuum insulated containers are designed to conserve the thermal energy already present in the contents of the container. The present invention, on the other hand, actually transfers thermal energy into the contents, adding to the thermal energy of the contents, and thereby keeping the contents at a higher temperature for extended periods of time.
The concept of a container which can be preheated to add thermal energy to the contents is not new. For example, U.S. Pat. No. 4,567,877, to SEPAHPUR, discloses a heat storage food container, adapted to be used in microwave ovens, using wet sand as a thermal storage medium. However, an obvious drawback to the SEPAHPUR container is that water undergoes a phase transition (from liquid to gas--vaporization) upon heating in the temperature range necessary for food preparation. Upon the phase change from liquid to gas, the molecules occupy a significantly greater volume, and consequently, the heat storage container must be engineered to structurally accommodate such changes.
The vaporization problem is addressed in BALDWIN, U.S. Pat. No. 5,601,744, which discloses a beverage container comprising an inner vessel with bottom and side walls, an outer wall at least practically surrounding the inner vessel, a chamber defined by the space between the inner vessel and the outer wall, and a microwave receptive material disposed within the chamber. Either or both of the inner vessel or the outer wall are transparent to microwave radiation and has a melting point greater than a melting temperature, and less than a vaporization temperature, of the microwave receptive material. Thus, the container is designed to melt before pressure is allowed to build within the closed space containing the microwave receptive material.
In addition to problems caused by vaporization, problems are also created when microwave susceptible materials undergo phase changes from solid to liquid. In these cases, it is necessary to engineer the container and/or material so as to contain the microwave susceptible material upon melting. ZIELINSKI et al., U.S. Pat. No. 5,520,103, discloses a heat retentive server comprising upper and lower shells, which include a thermoplastic material and are joined together to define a cavity therebetween. A heat storage medium comprising a phase-change material substantially fills the cavity and is unrestrained therein. The melting problem is addressed in ZIELINSKI et al. by use of a material which forms a gel at elevated temperatures.
Commonly assigned co-pending U.S. application Ser. No. 08/781,630, filed Jan. 10, 1997, discloses microwaveable heat retentive receptacles utilizing microwave absorbing materials. This application is hereby incorporated by reference as though set forth in full herein.
Containers for use in microwave ovens such as that disclosed in U.S. application Ser. No. 08/781,630 are formed from polymers and are substantially transparent to microwave radiation. By "substantially transparent to microwave radiation" herein is meant that an object formed of such a material and subjected to microwave radiation of about 1000 watts for a period of about 2 minutes will exhibit a change in temperature of about 5.degree. F. or less. Typical materials meeting this definition include polyolefins, e.g., polyethylene and polypropylene.
DOBRY, U.S. Pat. No. 4,937,412, discloses a method of heating a load object comprising the steps of using a microwave susceptible material which is fluid at a predetermined elevated operating temperature, enclosing the material in a means of containment, exposing the material to microwave radiation to generate heat and store it in the material and in the means of containment, and placing a load object in proximity to the means of containment. The means of containment may be either closed and flexible, or porous and permeable. If the latter, then the microwave susceptible material is held therein by capillary action.
U.S. Pat. No. 5,052,369, to JOHNSON, which is hereby incorporated by reference as though set forth in full herein, also discloses a heat retaining food container adapted for microwave use. Unlike SEPAHPUR, the microwave absorbing material of JOHNSON is a mixture of micro crystalline wax which exhibits a fusion temperature (melting point) between 175.degree. F. and 300.degree. F. The melted material is contained by the use of a thin film or a pouch-like enclosure.
Others have addressed the melt-containment problem by using materials that do not undergo a solid to liquid transition at higher temperatures. For example, U.S. Pat. No. 4,983,798, to ECKLER et al., discloses the use of materials which undergo a "mesocrystalline" phase change in the solid state prior to melting, such as pentaerythritol and neopentylglycol. ECKLER et al. is hereby incorporated by reference as though set forth in full herein.
BENSON et al., U.S. Pat. No. 4,572,864, discloses a composite material for storage of heat energy. The material comprises a polyhydric alcohol or derivative which is capable of undergoing a solid-state crystalline phase transformation. Such materials include pentaerythritol, pentaglycerine, neopentyl glycol, tetramethylol propane, monoamino pentaerythritol, diamino pentaerythritol, tris(hydroxymethyl)acetic acid, and mixtures thereof. The composite material also comprises materials from the group which includes metals, plastics, natural or artificial fibers, and porous rock. Also disclosed is a method of impregnating the phase-change materials into certain solid substances, including porous, fibrous, and stone-like materials.
CHAMBERLAIN et al., U.S. Pat. No. 5,294,763, discloses a heat susceptor comprising microwaveable heat susceptor particles in a matrix which is substantially nonreflective (i.e., "capable of transmitting microwave energy") to microwave energy. The particles comprise substrates which, like the matrix, are non-reflective of microwave energy. The particle substrates are coated with a material capable of absorbing microwave energy and converting it to heat. Materials for the matrix include ceramics and polymers. Materials for the substrate include glass and ceramics. Coating materials include electrically conductive and semi-conductive materials, such as metals and metal-containing compounds.
KREIBICH et al., U.S. Pat. No. 4,259,198, discloses a method of using crystalline resins as heat accumulators. The crystalline synthetic cross-linked resins used, which include polyester resins, further include crystallite-forming blocks linked to the resin through ether or ester linkages. The crystallite forming blocks comprise polymethylene chains which alternate with ether or ester linkages, and have at least thirty carbon atoms.
Commercially available standard microwave ovens for domestic use in the home typically are rated as having an "output" of, for example, on the order of from about 600 to about 1,000 watts. Typical commercial "convenience" foods are specifically designed to be heated to or near a desired or serving temperature (e.g., perceived to be desirable by the typical adult) in from about 2 to about 6 minutes. Consequently, it is believed that typical users of microwave ovens in domestic settings desire or expect to employ a microwave oven to heat the contents of a container to a desired serving temperature in a period of time of from about 2 to about 6 minutes.